The present invention relates generally to a portal radiation monitor and more particularly to a portal radiation monitor utilizing pulse shape discrimination to improve sensitivity and reduce false alarms.
It is important for the welfare and continued existence of mankind for controls to be maintained over the transportation and possession of special nuclear materials (SNM) such as .sup.235 U and .sup.239 Pu. Portal radiation monitors are often used to survey personnel leaving an institution containing special nuclear materials to ensure that these materials are not being secreted outside the facility.
A portal radiation monitor consists of three components: a detector, a signal processor and an alarm module. The detector is usually a scintillator which takes the form of an inorganic crystal such as NaI(T1), an organic liquid such as NE-235 or an organic solid such as BC-400 plastic. In order to maintain a reasonable detector cost it is also desirable to cover as much area as possible with a minimum of photomultiplier-scintillator units. For example, one commercially available portal monitor utilizes two vertical solid slab BC-400 (formerly NE-102) plastic scintillators, each scintillator being approximately six feet long and mounted on one side of a portal.
In most existing systems, the output of the photomultiplier tube is processed according to the amplitude of each pulse. The alarm module employed in these systems cannot verify that the processed signals are from radiation-induced events. As a result, such systems have high and unpredictable false-alarm rates and are generally not too sensitive.